There is a very easy way to remove those "weird genres"
In the tree under Audio there is a Library View called "Genres" if you expand it you will see a list of all the Genres Present in the Database/Tags
Which is just
one way to re-tag your files. There are many ways to mass-edit tags. You can also do it in any Pane-style View as well. Right-click on one of the values under the Pane for the tag you want to change and choose
Rename. Type in whatever you want, or blank to reset it back to Unassigned.
This is the same as selecting the item, selecting all of the files, and changing the tag in the Tag Action Window, though it has the benefit that it lets you operate on just that value (leaving other values for list-type fields, like [Genre], intact).